Sometimes, a lesson that takes the whole 90 minutes for one class period won't take very long at all in another. Luckily, with FL, we teachers have a lot of quick and easy review games that we can play. If we have an extra 15 minutes, we can play "matamoscas," practice with whiteboard, play the slap game, pictionary, charades, etc. In general, shorter lessons haven't really been a problem for us. I think that it would be harder in an English or Social Studies class but for us, it's no problem at all.